Before RF, you couldn't queue for extremes in the DF unless you had a premade full party anyway when the EX trial just came out. They only allowed solo queueing a couple months later. So it being in the RF was somewhat of an improvement for solo queuers, maybe they kept some of them in there too long though.
3.1 (Thordan): The quests "The Limitless Blue (Extreme)" and "Thok ast Thok (Extreme)" have been adjusted as follows:
Players may now queue solo via the Duty Finder. (Ravana, Bis)
3.2(Sephirot): Players can queue solo for The Minstrel's Ballad: Thordan's Reign via the Duty Finder.
3.3(Nidhogg): Players may now queue solo for Containment Bay S1T7 (Extreme) via the Duty Finder. (Sephirot)
3.4(Sophia): RF (Sophia EX), Thordan+ moved.
Starting with Sophia EX in RF, you could queue as solo as soon as patch day. Before that the policy they settled on was they wouldn't even add a solo queue until they added echo.